I think the tiebreaker is that two-party systems, left alone long enough, will tend to result in a cycle of (1) consolidation, (2) polarization, (3) constitutional hardball, and then (4) (usually) one-party dominance.
At the outset, FPTP doesn't immediately result in two parties. Early party systems are plagued by chaos because the earliest political alliances are the most likely to be alliances of convenience, which quickly fall apart - cf. Mike Duncan's "entropy of victory".
The chaos drives party leaders to consolidate power. Eventually, two relatively stable coalitions arise, and with the local/regional one-party dominance that FPTP tends to result in, each party ends up controlling the electoral machinery necessary to fend off challenges and process internal dissent without crackups.
With power consolidated, and weakening mobility of minor factions defecting between the two parties, polarization increases. Once the polarization reaches a nearly even 50/50 split of national equilibrium, parties stop playing traditional FPTP/two-party wedge politics (not enough wedges left!), and start playing constitutional hardball.
* In America, the constitutional hardball has led to a unique situation where elites are reluctant to capitalize on slim majorities for fear of lacking the legitimacy to make big changes. Thus, we've been muddling along in this stage for longer than usual.
At this point, the outcomes become unpredictable. Usually, one party will capitalize on a temporary majority to rig the entire national electoral system in its favor, and dominates as an illiberal democracy for a few generations. Sometimes, a democratization happens and reinvigorates the system - this happened in America's Civil Rights Movement - but this only lasts for so long. And sometimes, usually in smaller nations, it's actually possible for the people to realize the problems with FPTP, and successfully throw off the two party system and convert to a multiparty democracy.
* Quick note: Sometimes parties skip directly from an early temporary supermajority to consolidating one-party power. A *lot* of FPTP states end up like this.
This is just my opinion, but history is littered with examples of this happening. There are, of course, exceptions, especially when strong but dispopulous regional identities create durable spoiler parties that keep a two-party system from consolidating, but I think overall there's just too much evidence.
I also think that the consolidation, polarization, and one-party dominance stages are what give two-party systems the illusion of stability. At any given time, they may indeed seem to be relatively healthy. And the variability of outcomes means that the transitions between stages can be quite abrupt or hard to identify if you don't know what you're looking for - in the US, for example, up until recently we've had long periods of national one-party dominance whose occasional flips masked the consolidation that was really happening. Conversely, the back-and-forth of the polarization era that preceded today's constitutional hardball was actually *applauded* as a sign that America was "becoming more competitive". And yet, now having experienced polarization morph into hardball, it's hard to say that any systemic political developments in the past 70 years have really been *all that good*.
From The Comments: The Two-Party Doom Cycle
From The Comments: The Two-Party Doom Cycle
From The Comments: The Two-Party Doom Cycle
In Response To…
I think the tiebreaker is that two-party systems, left alone long enough, will tend to result in a cycle of (1) consolidation, (2) polarization, (3) constitutional hardball, and then (4) (usually) one-party dominance.
At the outset, FPTP doesn't immediately result in two parties. Early party systems are plagued by chaos because the earliest political alliances are the most likely to be alliances of convenience, which quickly fall apart - cf. Mike Duncan's "entropy of victory".
The chaos drives party leaders to consolidate power. Eventually, two relatively stable coalitions arise, and with the local/regional one-party dominance that FPTP tends to result in, each party ends up controlling the electoral machinery necessary to fend off challenges and process internal dissent without crackups.
With power consolidated, and weakening mobility of minor factions defecting between the two parties, polarization increases. Once the polarization reaches a nearly even 50/50 split of national equilibrium, parties stop playing traditional FPTP/two-party wedge politics (not enough wedges left!), and start playing constitutional hardball.
* In America, the constitutional hardball has led to a unique situation where elites are reluctant to capitalize on slim majorities for fear of lacking the legitimacy to make big changes. Thus, we've been muddling along in this stage for longer than usual.
At this point, the outcomes become unpredictable. Usually, one party will capitalize on a temporary majority to rig the entire national electoral system in its favor, and dominates as an illiberal democracy for a few generations. Sometimes, a democratization happens and reinvigorates the system - this happened in America's Civil Rights Movement - but this only lasts for so long. And sometimes, usually in smaller nations, it's actually possible for the people to realize the problems with FPTP, and successfully throw off the two party system and convert to a multiparty democracy.
* Quick note: Sometimes parties skip directly from an early temporary supermajority to consolidating one-party power. A *lot* of FPTP states end up like this.
This is just my opinion, but history is littered with examples of this happening. There are, of course, exceptions, especially when strong but dispopulous regional identities create durable spoiler parties that keep a two-party system from consolidating, but I think overall there's just too much evidence.
I also think that the consolidation, polarization, and one-party dominance stages are what give two-party systems the illusion of stability. At any given time, they may indeed seem to be relatively healthy. And the variability of outcomes means that the transitions between stages can be quite abrupt or hard to identify if you don't know what you're looking for - in the US, for example, up until recently we've had long periods of national one-party dominance whose occasional flips masked the consolidation that was really happening. Conversely, the back-and-forth of the polarization era that preceded today's constitutional hardball was actually *applauded* as a sign that America was "becoming more competitive". And yet, now having experienced polarization morph into hardball, it's hard to say that any systemic political developments in the past 70 years have really been *all that good*.
Anyways, that's just me.